rust: cpumask: rename CpumaskVar::as[_mut]_ref to from_raw[_mut]

The prefix as_* shouldn't be used for constructors. For further
motivation, see commit 2f5606af ("device: rust: rename
Device::as_ref() to Device::from_raw()").

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>